Technical Field
The present description relates to techniques for the estimation of the heart-rate.
Various embodiments may apply, e.g., in wearable, in particular wrist-wearable devices, for continuous monitoring of the heart-rate in fitness/wellness applications.
Description of the Related Art
Among the methods for the estimation of the heart-rate in a subject, one of the most employed makes use of optical means to detect heart beat to evaluate the heart-rate, based on photoplethysmography (PPG). Photoplethysmography involves obtaining optically a volumetric measurement of an organ (plethysmogram). A photoplethysmogram is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.
PPG was historically first employed with finger clips in medical applications. Lately PPG has been employed also on wrist, arm, forearm, to make it suitable for fitness applications.
The PPG techniques typically allow an easy estimation of the heart rate at rest. In motion conditions the PPG signal is affected however by a very low SNR (Signal to Noise Ratio) and SIR (Signal to Interference Ratio). In this specific context SNR means the ratio of the heart rate, e.g., the signal S indicative of the heart rate, to all the other signals, e.g., the noise N. It can be measured by having a subject wearing a cardio frequency meter and a PPG device. The power of the frequency component (peak) measured by the PPG device closest to cardio frequency measured by the cardio frequency meter is the signal S. The total power T is T=(S+N). Thus, for the SNR it is S/N=S/(T−S). The SIR value is obtained as ratio of the signal S over I, the power of the strongest non-cardiac frequency component.
Poor performances are sometimes obtained by processing a PPG signal with known frequency-domain techniques or time-domain techniques, including adaptive filtering techniques.
Thus problems affecting the PPG are strong motion artifacts leading to low SNR, low SIR due to motion artifacts, spikes in the detected signal due to motion.
In the state of the art it is known, in order to compensate the above discussed effects of motion on SNR and SIR and other aspect of the signal detection, to acquire optically from the body organ a heart rate signal, acquire an acceleration signal representative of the acceleration of such body organ, selecting data blocks of said acquired heart rate signal and acceleration signal, compensating in the time domain the heart rate signal by the acceleration signal.
The following publications described similar prior art techniques for the reduction of the artifacts:
Yuta Kuboyama, “Motion Artifact Cancellation for Wearable Photoplethismography Sensor”, MIT, 2009;
K. Ashoka eddy, V. Jagadeesh Kumar, “Motion Artifact Reduction in Photoplethysmographic Signals using Singular Value Decomposition”, Instrumentation and Measurement Technology Conference, Poland, May 1-3, 2007;
H. Han, M. Kim, J. Kim, “Development of real-time motion artifact reduction algorithm for a wearable photoplethismography”, Proceedings of the 29th Annual International Conference of the IEEE EMBS Cite Internationale, Lyon, France, Aug. 23-26, 2007;
P. Wei, R. Guo, J. Zhang, Y. T. Zhang, “A New Wristband Sensor Using Adaptive Reduction Filter to Reduce Motion Artifact”, Proceedings of the 5th International Conference on Information Technology and Application in Biomedicine, Shenzen, China, May 30-31, 2008.